This method is directed to marine seismic data acquisition systems and, in one aspect, such systems with a streamer, streamers, and/or one or more seismic sources towed by one or more powered vehicles. This invention is related to the field of Common Mid-Point (CMP) marine seismic data acquisition.
In the field of marine seismic data acquisition, seismic signal sources, e.g. air guns, are towed behind a vessel, which may also tow a set of seismic sensors. The sensors are normally contained in streamers that are comprised of seismic signal receivers, e.g. hydrophones, which are sensitive to reflections and refractions from sound impulses emitted by seismic sources.
It has long been known that for high quality data, multiple samplings of the same subsurface areas are required. The addition of multiple signals from the same subsurface reflection point results in an increase in the information in signals along with a cancellation of the noise. This process is commonly known as the Common Mid-Point method.
FIG. 1A shows schematically a marine seismic survey system. A ship 14 tows a seismic source 5 of any known type, for instance air guns. Activating or firing a seismic source initiates acoustic energy which propagates from the source through the water and the earth. Also towed by ship 15 in this example are one or more sets of seismic sensors 13 or seismic receivers usually referred to generally as streamers. Acoustic energy propagated through the earth from subterranean features and may be detected by seismic sensors. The seismic sensors in the streamers may be single sensors or groups of sensors. The same vessel often tows streamers and sources, but this varies with survey acquisition logistics. Streamer 13 contains, as an example for the illustration, 15 sensors or groups of sensors; sensor 11 is closest to source 5 while sensor 19 is furthest away. Modem streamers may contain hundreds of sensors or sensor groups. Modem seismic acquisition systems may contain many streamers and several sources all in operation to acquire data in an area.
FIG. 1B shows how seismic energy propagates from the seismic source 5 to the streamer 13 sensors. Each time a seismic impulse is generated by a source, there is a corresponding midpoint (or Common Mid-Point, CMP) for every seismic sensor in a streamer. Seismic energy generated by source 5 travels to midpoint or CMP 1 and is reflected back to seismic sensor 11, the nearest sensor to source 5 of streamer 13. Seismic energy also reflects off of CMP 2 and travels to seismic sensor 19, the furthest sensor from source 5 of streamer 13. Of course, there are midpoints or CMPs corresponding to each seismic source impulse with each of the streamer 13 sensors. For example, the streamer shown in FIG. 1B contains 15 seismic sensors, and so each seismic impulse generated by source 5 would produce 15 CMPs from CMP 1 to CMP 2 inclusive.
FIG. 1C shows how the addition of another seismic source 7 towed, for example by ship 18, will produce CMPs from a source following the acquisition system in the direction of survey as demonstrated in FIGS. 1A and 1B. Here, the seismic energy travels from seismic source 7 to CMP 4 and back to the seismic sensor 19 nearest the source 7. Seismic energy travels from source 7 to CMP 3 and then the seismic sensor 11 furthest from source 7. Again, there are CMPs for each seismic sensor and seismic source pair, each time a source generates a seismic impulse.
As is known in the art, the number of times the same subsurface mid-point areas are sampled is commonly referred to as the xe2x80x9cfoldxe2x80x9d of the survey. A uniform distribution of fold over a survey area is desirable. A method or system that will lead to efficient acquisition of seismic surveys having a uniform distribution of fold is desirable. Uniformity in fold distribution results in a more even and predictable subsurface resolution, and therefore, seismic surveys with a more uniform fold distribution are desirable and operational changes that result in a haphazard or unpredictable fold distribution are not desirable.
Another way to improve subsurface resolution is to increase the areal resolution of a seismic survey by moving sampling bins closer together by decreasing the size of the subsurface sampling areas known as midpoint or CMP bins. Subsurface sampling areas (e.g. CMP spacing) need to be made as small as operational and economic constraints will allow.
The recording systems used for marine seismic data acquisition normally allow firing of shots based on distance, the Distance Shooting method, or based on time, the Timed Shooting method. For geophysical reasons (for instance, fold uniformity) the preferred option is to fire shots based on distance. However, when currents in the seismic survey area are too strong the Distance Shooting method may be problematic to implement and up to half of the expected shots may be missed. Under strong current conditions the timing method may be preferred. Recording systems are set up to handle either the xe2x80x98distance shooting methodxe2x80x99 or the xe2x80x98timed shooting method,xe2x80x99 but not both at the same time. Recording systems must normally not be acquiring data for a switch between one system and the other to be made. This results in an interruption of the survey acquisition program and can be an inefficient use of resources.
Marine seismic operations are often large, complex and expensive. There are recognized needs for improving the efficiency of seismic field operations. A system and method that will increase survey efficiency and streamline operational logistics is highly desirable. The present invention addresses these recognized needs.
Accordingly, there is a need for a method and system to acquire seismic data efficiently without having to interrupt field operations to change from one acquisition method, for example based on timed shooting, to another method, for example based on distance or position shooting. The prior art does not disclose systems or methods to improve the efficiency of the use of acquisition equipment as embodied in the present invention.
The present invention, in certain embodiments, discloses a method for acquiring marine seismic data using a seismic acquisition system where shot points are determined and shot records recorded. A time at which the source vessel will be substantially at a shot point position is predicted and the seismic source is activated at the shot point. The shot is recorded and the next location for recording a shot is determined. If the source vessel has passed the determined next shot point position, the seismic source is activated and a shot is recorded. If the source vessel has not reached the next shot point, a time delay is implemented until the shot point is reached. The method allows for efficient acquisition of seismic data under varying field conditions with minimal operational interruptions by acquiring data using a distance based shot point determination method unless a time based acquisition method becomes more appropriate due to field conditions or other operational considerations.